This invention relates to improvements in magnetically actuated reed switch devices, and particularly to those using permanent magnet biasing means for controlling the sensitivity of a reed switch to changes in surrounding magnetic flux density.
A reed switch comprises a cylindrical glass capsule containing electrical contacts attached to magnetic material, that is, material which becomes magnetized in the presence of a magnetic field, at least one such contact being carried on a movable end of a flexible blade, or "reed", of magnetic material.
Such a reed switch is operated by increasing the magnetic flux density in the vicinity of its magnetic portions which induces increased magnetism in those portions, thereby causing the reed to be attracted toward another magnetic component of the reed switch by the magnetism. When the magnetic flux density is sufficiently reduced, the reed is released and the reed switch resumes its normal condition. In various forms of reed switches the electrical contacts may move either to close or open an electrical circuit under the influence of a magnetic field in a single-pole-double-throw switch, or the reed-carried contact may move from a closed position with respect to a non-magnetic fixed contact to a closed position with respect to a magnetic fixed contact in a single-pole-double-throw switch.
In security systems, such magnetic reed switches have previously been used in conjunction with an actuating permanent magnet, which, when close enough to a reed switch, provides the increase in magnetic flux density to actuate it. The sensitivity of a reed switch to such actuation is limited, however, and use of such a reed switch in a security system application requires either a fairly strong actuating magnet or an installation providing a very small gap between the actuating magnet and the sensing reed switch.
Normally the security device reed switch and its associated electrical conductors leading to a security system control unit and alarm device are mounted in or on the frame surrounding a doorway or window opening, and the actuating magnet is located in or on the door or window sash so that movement of the door or window from a predetermined position moves the actuating magnet and thereby allows the reed switch to release, or resume its normal condition, producing an electrical signal detected by the monitoring security system control unit.
This application of reed switches has in the past required extremely careful alignment between the switch and the actuating magnet. The low sensitivity of the reed switch has caused the reliability of such security switches to be less than desired, since a slight movement of the actuating magnet could allow the magnetically held contacts to be released, particularly in environments including magnetic materials such as steel fire doors. For example, reed switches installed to monitor a door might produce false alarms if wind gusts cause the door to shift slightly, or switches installed to monitor overhead or sliding doors can produce erroneous indications of the position of such doors because of minor misalignment of such doors in their tracks.
A characteristic of reed switches, known as hysteresis, is that once an actuating magnet has approached a reed switch closely enough to cause actuation, the switch will remain magnetically actuated as the actuating magnet is withdrawn to a greater distance before the switch releases or resumes its normal state. This characteristic is important because it is usually desirable in security applications to have actuation and release occur at as nearly as possible the same point, particularly where there is little relative motion between an actuating magnet and a switch.
For example, when a security switch and its actuating magnet are mounted in the hinge sides of a door and doorway respectively, the switch should release before the edge of the door opposite the hinges is clear of the doorway. Also, it is desirable to mount the switch and actuating magnet in one of the hinges, to make installation simple and unauthorized actuation more difficult. The characteristic hysteresis separation between actuation and release points of ordinary magnetic reed switches, however, makes such installation less sensitive to small movements than is desirable.
The use of larger actuating magnets mounted on doors and windows to allow insensitive reed switches to remain actuated despite small movements of doors and windows reduces the number of false alarms, but makes such a magnetic switch more easily detected and located by a magnetometer and thus less secure from tampering. Moreover, larger actuating magnets also increase the effect of hysteresis. Additionally, the material used in producing the actuating magnets is not inexpensive, and larger magnets appreciably increase the cost of the devices.
As in security systems, magnetically actuated switches are useful in relay systems controlling machines, and problems similar to those occurring in a security system also occur in that environment.
One means of increasing the sensitivity of such a reed switch is to place a permanent magnet near the switch to bias the reed by providing part of the magnetic flux density necessary for actuation. Permanent magnet bias means for controlling the sensitivity of reed switches have been previously disclosed, for example, by variation of the distance or angular relationship between the magnet and the longitudinal axis of the reed of the switch, as shown in Nicholls U.S. Pat. No. 3,974,469, and by varying the location of the reed along an imaginary axis parallel to the axis of polarity of the biasing magnet, as shown by Tann U.S. Pat. No. 3,305,805. However, the device of the Nicholls patent requires either a threaded hole through the biasing magnet, in order to vary its distance, or a disc-shaped rotatable magnet and socket to vary its angular relationship, both of which are somewhat expensive and complicated means of attachment, and Tann shows no apparatus for holding the axis of polarity of a magnet parallel to the reed of a reed switch. Also, while Tann shows a means of adjustment of the position of a reed switch relative to a magnet, only discrete, not continuous variations of position are provided. Thus a need exists for an inexpensively manufactured, magnetically actuated proximity switch which has continuously adjustable sensitivity and is particularly suitable for use in physical security monitoring systems or position control systems.